In a typical radio communications network, communication devices, also known as wireless terminals, mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a control node such as a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. The base stations communicate over the air interface operating on radio frequencies with the communication devices or user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for communication devices or user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base stations that do not report to RNCs.
Fundamental cellular communication occurs between one or more communication devices and a network, so that transmitted data always is routed via the base station. The base station includes functionality that ensures that the radio resource is used as efficiently as possible, by scheduling communication device transmissions based on some suitable metric.
There are reasons why communication directly from one communication device to another, i.e. without passing by a base station may be provided. Reasons could e.g. be that the base station is not working properly, or that direct communications are needed within a small area in an emergency situation or similar. Such direct communication between communication devices is often referred to as Device-to-Device (D2D) communication. In existing D2D concepts and technology components, a D2D layer may use cellular uplink (UL) and/or downlink (DL) resources that may overlap with radio resources used for supporting cellular traffic. The radio resources used by the D2D layer includes the physical resource blocks (PRB) that are used for control of the D2D traffic by the cellular base station (BS) such as an LTE eNB, the PRBs used by the D2D traffic, and the PRBs used for neighbor, or peer, discovery by a D2D capable communication device or user equipment.
In 3GPP D2D is called Proximity Services (ProSe); i.e. services that can be provided by the 3GPP system based on communication devices or UEs being in proximity to each other, i.e. receiving communication devices are within a distance from the transmitting communication device so that they can receive the transmissions. Architectures and reference models are described in TS 23.303 v. 12.0.0. The interface between communication devices in ProSe is called PC5 interface. The air or radio interface between an eNB and a communication device is called Uu interface.
D2D Data Transmission Procedure
The purpose of the data transmission procedure is to convey user data from one communication device, UE-A, to another communication device, UE-B. Investigations have shown that in order to meet the requirement on coverage, each D2D transport block needs to be transmitted four times. FIG. 1 shows an overall transmission procedure for D2D transmission with network (NW)-controlled resource allocation. The transmission procedure for D2D generally follows the procedure for legacy transmissions; the UE-A sends a scheduling request (SR) over Physical Uplink Control Channel (PUCCH) to the eNB, the eNB sends a grant to the UE-A over Physical Downlink Control Channel (PDCCH). For D2D the communication device, UE-A, further sends a buffer status report (BSR) for D2D (D2D BSR) with information on, among others, amount of data, over Physical Uplink Shared Channel (PUSCH); the eNB grants the communication device resources for transmission by transmitting a D2D grant.
The purpose of the D2D-BSR is to inform the eNB about the amount of data the UE has on logical channels related to D2D. Although this makes it possible to reuse the existing BSR, it would require at least one logical channel group for D2D communication. If the UE is also configured with legacy LTE bearers and D2D discovery, the four existing logical channel groups may become a restriction.
For D2D the eNB could set up periodic BSRs related to the validity time of the D2D grant for increased efficiency. It should be noted that the D2D-BSR is transmitted on the Uu interface and not on PC5 interface.
The D2D grant should be transmitted on the PDCCH similar to legacy PUSCH grants. The purpose of the grant is to allow the communication device to transmit data on the ProSe physical channel. The grant also allows the eNB to control which communication device gets to transmit when and on which radio resources. This reduces interference and the risk for collisions. A Scheduling assignment indicating radio resources for the D2D communication may further be informed back to the eNB over D2D physical channel (phy).
Before the communication device UE-A can transmit a Scheduling Assignment (SA) the UE-A needs to have a valid grant, FIG. 2. In FIG. 2 it is shown that the communication device, UE-A, performs a request and grant procedure with the eNB. This is followed by a scheduling assignment procedure between the communication devices and finally transmissions (TX) of data denoted Data 1-8 TX procedures, between the communication devices are performed. The Data 1-8 TX procedures allow for data to be sent just once, as well as to be repeated up to 7 times, thus allowing a total of 8 transmissions per cycle.
In the example in FIG. 3 assuming a configuration allowing a total of 4 transmissions per cycle, an SA cycle of a scheduling assignment procedure is 160 ms. In each SA cycle there are up to 4 occasions for a transmission of the SA. So, in short, every 40 ms there is an opportunity to send an SA, a next SA occasion. By having up to 4 opportunities a communication device may send an SA in one occasion and listen for other SAs in the same cycle. This means that a communication device can send and receive D2D transmissions continuously, if the transmission patterns are orthogonal i.e. transmission patterns of the D2D transmissions do not overlap in time, i.e. are sent in a Time Division Multiplex (TDM) fashion. The TX communication device sends an SA with control information, before sending actual data. In coverage the control information is based on information from a scheduling grant. Out of coverage the control information is pre-configured. The receiving communication device only needs to listen for the SA. From the control information in the SA the RX communication device knows on what resources to look for data. The purpose of the Scheduling assignment is twofold.
1) It allows the communication device to only track the SA and perform Discontinuous Reception (DRX) in-between.
2) It contains information on how to decode the data, e.g. which exact time frequency resource has been/will be used.
The different types of D2D traffic, e.g. control plane, user plane and discovery, impose an extra load on the radio resources that is not present in the radio communications networks that do not support D2D communication. E.g. prior to every transmission of data over a PC5 link, an SA needs to be transmitted with information on, e.g., radio resources to listen to in order to be able to decode data. Hence, the D2D communication increases the load in the radio communications network reducing the performance of the radio communications network.